1. Field of the Invention
The present invention relates to a semiconductor memory device such as EEPROMs of the NAND-cell, NOR-cell, DINOR (Divided bit line NOR)-cell and AND-cell types, and more particularly to a semiconductor memory device having an improved sense amplifier of the current sense type.
2. Description of the Related Art
A sense amplifier in a semiconductor memory device such as a flash memory basically senses the presence/absence or the level of cell current flowing in accordance with data in a memory cell, thereby deciding the value of data. The sense amplifier is usually connected to a bit line (data line) to which a number of memory cells are connected. The sensing scheme is roughly divided into the voltage sense type and the current sense type.
A sense amplifier of the current sense type precharges a bit line isolated from the memory cells to a certain voltage, discharges the bit line through the selected memory cell, and detects the discharged state of the bit line at a sense node connected to the bit line. At the time of data sensing, the bit line is isolated from the current source load to detect the bit line voltage determined from cell data.
A sense amplifier of the voltage sense type, on the other hand, supplies read current flowing in a memory cell via the bit line, thereby sensing data. Also in this case, cell data determines the bit line voltage, and eventually data determination at the sense node connected to the bit line detects a difference in voltage at the sense node based on the difference in cell current.
The sense amplifier of the current sense type and the sense amplifier of the voltage sense type have the following advantages and disadvantages in general. The voltage sense type utilizes charging and discharging bit lines and accordingly has less power consumption. In a mass storage memory with a large bit line capacity, though, charging/discharging is time-consuming and accordingly fast sensing becomes difficult. In addition, the amplitude of the bit line voltage is made relatively large in accordance with cell data and accordingly a noise between adjacent bit lines causes a problem.
In contrast, the sense amplifier of the current sense type senses data while supplying read current flowing in the memory cell via the bit line, thereby enabling fast sensing. In addition, a clamp transistor (presense amplifier) arranged between the bit line and the sense node is used to reduce the amplitude of the bit line voltage in accordance with cell data and accordingly the noise between bit lines hardly causes a problem. Also in this case, however, reading is executed on alternate bit lines, and other bit lines not subjected to reading are grounded and used as shields to exclude influences between bit lines on reading.
To the contrary, the bit line potential may be controlled such that it is always fixed at a constant voltage to exclude influences between bit lines and allow all bit lines to be sensed in parallel on sensing. Such a sense amplifier of the ABL (All Bit Line) type has been proposed (JP 2006-500729T, paragraphs 0076-0079, FIG. 13).
In such the sense amplifier of the current sense type, however, the power consumption becomes larger than the sense amplifier of the voltage sense type by the extent of the current flowing while sensing. In recent, years, advanced fine patterning of devices results in a larger bit line capacity, and accordingly charging the bit line consumes a larger amount of current as a problem. The voltage sense type can restrict the current with the current control circuit on charging the bit line. In contrast, if the current sense type uses a power source restricted by the current control circuit, a bit line current can not be determined from the threshold of a cell transistor when current flows in all bit lines. In this case, the bit line current can be determined from the load in the current control, circuit, resulting in failed sensing. Therefore, the sense amplifier of the current sense type requires direct connection of the power source not via the current control circuit and allows a larger amount of current to flow on charging the bit line as a problem. In particular, the beginning of initial charging immediately after the initial charging circuit in the sense amplifier is connected to the bit line allows an excessive initial current to flow.